Earth used to rotate very rapidly, some say (5)
I believe the answer is:
world
'earth' is the definition.
(I know that earth can be written as world)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for world that I've seen before include "The globe" , "All humankind" , "Earth - universe" , "Planet like the 7 down" , "our home" .)